1. Field of the Invention
This invention relates to polyacrylamide gels as used in slab gel electrophoresis.
2. Description of the Prior Art
When electrophoresis is performed in a slab gel, several samples can be analyzed simultaneously in the same gel and the resulting electropherograms can be observed and read visually by identifying the locations of the bands on the gel that correspond to the individual components. Polyacrylamide is a gel material that is widely used in slab gels.
Slab gels are frequently supplied in pre-cast form in cassettes that typically contain two flat transparent plates with the gel retained between them. The plates may be glass or plastic, one commonly used plastic being a polystyrene-acrylonitrile blend. A difficulty with certain pre-cast polyacrylamide gels is that during storage the gels appear to separate from the cassette plates. This creates a pathway between the gel and one or both of the plates in which the sample can migrate during electrophoresis. This migration causes shadow bands in the electropherogram which obscure the clarity and identification of the parent bands, i.e., those that are formed as a direct result of the electrophoretic separation. Shadow bands occur most frequently in pre-cast gels that have been stored without cooling.
Another problem encountered with polyacrylamide slab gels is a tendency of the gels to stick or adhere to the plates. This presents a difficulty once the separation is completed and the gel must be removed from the plates for purposes of staining, photographing or other observation, detection or recordation. Attempts to remove a gel that is sticking to one or both of the plates can result in a damaged gel and a ruined experiment. This problem is especially acute for gels of low concentration and for gels used for isoelectric focusing.
The polymerization reaction to form polyacrylamide is inhibited when dissolved oxygen is present in the gel-forming liquid at or near the gel plate. This is especially true when the gel plates are plastic, such as polystyrene-acrylonitrile, for example. To prevent this inhibition from occurring, a coating of polyvinylidene chloride or polyvinyl dichloride (PVDC) is often applied to the plates prior to contacting the plates with the polyacrylamide gel material. Unfortunately, these coatings exacerbate the sticking problem when the gel is an isoelectric focusing gel, for example one with a pH ranging from 5 to 8. In addition, electrophoresis images produced both with and without these coatings often contain irregularities that appear to be the result of a separation between the gel and the plate.